1. Field of the Invention
This invention relates to light emitting diode packages and displays utilizing light emitting diode packages as their light source.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Technological advances over the last decade or more has resulted in LEDs having a smaller footprint, increased emitting efficiency, and reduced cost. LEDs also have an increased operation lifetime compared to other emitters. For example, the operational lifetime of an LED can be over 50,000 hours, while the operational lifetime of incandescent bulb is approximately 2,000 hours. LEDs can also be more robust than other light sources and can consume less power. For these and other reasons, LEDs are becoming more popular and are now being used in more and more applications that have traditionally been the realm of incandescent, fluorescent, halogen and other emitters.
In order to use an LED chip in conventional applications it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical LED package/component 10 illustrated in FIG. 1, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to, or integral with the reflective cup 13. The reflective cup 13 may be filled with an encapsulant material 16 which may contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 14, which may be molded in the shape of a lens. While the reflective cup 13 may direct light in an upward direction, optical losses may occur when the light is reflected (i.e. some light may be absorbed by the reflector cup due to the less than 100% reflectivity of practical reflector surfaces). In addition, heat retention may be an issue for a package such as the package 10 shown in FIG. 1, since it may be difficult to extract heat through the leads 15A, 15B.
Different LEDs packages, such as those shown in FIG. 1, can be used as the light source for displays, both big and small. Large screen LED based displays (often referred to as giant screens) are becoming more common in many indoor and outdoor locations, such as at sporting arenas, race tracks, concerts and in large public areas such as Times Square in New York City. Some of these displays or screens can be as large as 60 feet tall and 60 feet wide. These screens can comprise thousands of “pixels” or “pixel modules”, each of which can contain a plurality of LEDs. The pixel modules can use high efficiency and high brightness LEDs that allow the displays to be visible from relatively far away, even in the daytime when subject to sunlight. The pixel modules can have as few as three or four LEDs (one red, one green, and one blue) that allow the pixel to emit many different colors of light from combinations of red, green and/or blue light. In the largest jumbo screens, each pixel module can have dozens of LEDs. The pixel modules are arranged in a rectangular grid. In one type of display, the grid can be 640 modules wide and 480 modules high, with the size of the screen being dependent upon the actual size of the pixel modules.
Most conventional LED based displays are controlled by a computer system that accepts an incoming signal (e.g. TV signal) and based on the particular color needed at the pixel module to form the overall display image, the computer system determines which LED in each of the pixel modules is to emit light and how brightly. A power system can also be included that provides power to each of the pixel modules and the power to each of the LEDs can be modulated so that it emits at the desired brightness. Conductors are provided to apply the appropriate power signal to each of the LEDs in the pixel modules.
LED displays are rarely mounted at the viewer's eye level, and are more typically mounted at an elevation above eye level, such as on the side of a building or the top of the grandstands in a stadium. Referring now to FIG. 2, a conventional LED display 20 is shown mounted at an elevated point above the eye level of the viewer 22. The viewer 22 is typically positioned below the display 20 and looks up to the display such that the viewer's line of sight 24 to the display 20 is at an angle θ to the display's perpendicular emission direction 26. The LED display in FIG. 2 typically comprises a plurality of emitters 28 such as those shown in FIG. 1 that exhibit a peak emission that is typically along the packages longitudinal axis with peak emission near the center.
Having a display comprising a plurality of LED packages 28 can result in display peak emission characteristics perpendicular in the perpendicular direction 26, as shown. The Iv and far field pattern (FFP) peak emission characteristics for the LED display 20 can be perpendicular to the display along the perpendicular axis 26. The viewer's line of sight 24 is below perpendicular when the display 20 is mounted at an elevated point, much of the light emitted by the display is not seen by the viewer and is wasted. This can be true for viewers below the display and the side of the display. One way to reduce the amount of light that is wasted is by mounting the display at an angle to better match the viewer's line of sight 24, but this can require complex and expensive mounting hardware that is difficult to use, particularly for very large displays mounted at high elevations.
Viewers are often not directly in front of an LED based displays when it is viewed. Depending on where the viewer is located the horizontal viewing angle can be different. Furthermore, when a person is moving by an LED display, such as walking by, it is viewed at many different horizontal angles. Typical LED displays with peak emissions near the center can experience a drop-off in emission intensity at different horizontal angles. The far field pattern (FFP) for the different LED packages in each of the pixels can also be different such that the LED display can experience image quality variations when viewed from different angles.